1. Field of the Invention
The present invention generally relates to the art of microcircuit device fabrication, interconnection, and testing, and more specifically to a method for attachment of multi-contact electrical components to substrates using ribbon bonding, which enables the components to be tested prior to final assembly.
2. Description of the Related Art
High density microcircuit devices, including hybrid microcircuits, may be fabricated by providing a substrate with a pattern of electrical interconnect metallizations formed thereon, adhering electrical components or devices to the substrate, and interconnecting contacts on the devices to corresponding contacts on the substrates by wire or ribbon leads. This has been conventionally accomplished by wedge or gold ball bonding, using pressure, heat, ultrasonic energy, and/or a combination thereof.
In wedge bonding, a tool is positioned such that a direction of supply of gold or aluminum wire or ribbon from a spool is aligned with a contact on a device and a corresponding contact on a substrate. The wire extends under the wedge bonding tool foot. The tool is positioned over the contact on the device and lowered to press the wire onto the contact for a predetermined dwell time and force to form an inner bond. The tool is then raised, moved into position over the contact on the substrate, and then lowered to press the wire onto the contact to form an outer bond. After forming the resulting interconnection, the tool is raised, and a clamp tugs off the wire from the second bond and shoves the wire back under the bonding tool foot, readying it for the next bond. The cycle is essentially the same for ultrasonic, thermocompression, or thermosonic wedge bonding.
Ball bonding is similar to wedge bonding except that gold wire is used exclusively, and the end of the wire is formed into a ball which is bonded into a "nailhead" configuration. A key advantage of ball bonding over wedge bonding is the freedom of movement of the bonding tool; it can be moved in any direction after the first bond is formed.
The increasing complexity of integrated circuit (IC) devices, especially hybrid microcircuits, and the density of multichip circuits, mandate electrical testing of individual components and devices prior to assembly. This is necessary to ensure acceptable yields and reduce costly fault isolation and replacement of bad devices. Without this ability, the overall cost of hybrid microcircuits would be increased due to extensive re-work cycles and the possibility of damaging substrates or adjacent devices.
One known approach to enable pre-testing of devices prior to final assembly is to utilize tape automated bonding (TAB). This process, as well as wedge and gold ball bonding, is described in a textbook entitled "HYBRID MICROCIRCUIT TECHNOLOGY HANDBOOK", by J. Licari et al, Noyes Publications, Park Ridge, NJ, 1988, pp. 191-208. A device is first bonded to a plated copper/polyimide tape carrier, and then electrically tested and burned in. After testing, the device on the tape is excised and assembled into a hybrid package.
The TAB process is subject to a number of limitations, including the necessity of providing dedicated and expensive equipment for each process step. A long lead time and high cost are associated with providing specially designed tape carriers for each IC type. The tape materials must be developed to meet burn-in requirements without warping or stressing inner bonds. Hard tooling is required to excise and form TAB leads. Multiple bonds are placed on the same IC bond pad to accomplish single point bumping and inner lead bonding. In addition, bumping is required to provide a bonding surface above the passivation layer, and ICs must be available in wafer form to plate the bump with a barrier layer.
A second approach to enable device testing prior to final assembly is to wire bond a device with long wires into a temporary test package. This is done with a regular wirebonder, and there is no special technology to hold the alignment of the wires. The device is tested, burned-in, and the wires are cut. The device is then mounted in the final hybrid microcircuit package, the wires are aligned to the hybrid contacts by hand, and are then bonded. This process is difficult (special operator skills are required), and it is not suitable for production. Also, the inner lead bond is not protected from extra stress throughout the process.